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Diffusion is a controlling factor in the development of microstructure. Another factor is solubility, which is a measure of how much of a particular element can be accommodated by the crystal lattice before it is rejected. In metals when two or more elements are soluble in the crystal lattice, a solid solution is created (somewhat analogous to a liquid solution of sugar in hot coffee).

For example, when added to iron, carbon has very limited solubility in ferrite but is about 100

times more soluble in austenite, as seen in the iron–carbon diagram in Fig. 2 (a limited

version of the diagram in Fig. 1). The maximum solubility of carbon in ferrite is about

0.022% C at 727 C while the maximum solubility of carbon in austenite is 100 times more,

2.11% C at 1148 C. At room temperature the solubility of carbon in iron is only about

0.005%. Any amount of carbon in excess of the solubility limit is rejected from solid solution

and is usually combined with iron to form an iron carbide compound called cementite. This

hard and brittle compound has the chemical formula Fe3C and a carbon content of 6.7%.

This is illustrated in the following two examples. The first example is a microstructure of a

very low carbon steel (0.002% C), shown in Fig. 3a. The microstructure consists of only

ferrite grains (crystals) and grain boundaries. The second example is a microstructure of a

low-carbon steel containing 0.02% C, in Fig. 3b. In this microstructure, cementite can be

seen as particles at the ferrite grain boundaries. The excess carbon rejected from the solid

solution of ferrite formed this cementite. As the carbon content in steel is increased, another

form of cementite appears as a constituent called pearlite, which can be found in most carbon